1. Field of the Invention
This invention relates to a current compensation drive circuit for an electrical motor the concepts of which may be applied to a motorized electrical hand tool.
2. Related Art
Generally, DC motors are driven with a constant voltage source if a constant motor speed is desired out of them. For motors with small winding resistances or with low mechanical loads, this may be sufficient for the application. For instance, a 24 volt compressor motor rated with a 3 amp draw and a 0.61 ohm resistance will experience only a 1.8 volt winding drop at the rated current, which is 7.5% of the drive voltage. This may well be acceptable for the compressor.
Hand tools and, in the nomenclature of ophthalmic surgical instruments, handpieces often use electrical motors. In surgical hand tools, miniaturization is of primary importance and small motors have relatively large winding resistances. For instance, in an electrical vitrectomy handpiece, a motor with a 10 mm diameter must operate at a speed that requires a back electromotive force of 8 volts, a current draw of over 100 mA, and a winding resistance of 20 ohms. In this example, the volt winding drop is in excess of 2 volts, or more than 25% of the target back electromotive force. This quantity is significant, and compensating for the volt winding drop improves the performance of the hand tool significantly.
Motors for hand tools or handpieces operate at various speeds and encounter differing loads. For example, when a hand tool is operating without a load, the rotation of the motor achieves a certain speed. However, when the hand tool encounters a load, the rotational speed decreases to a lower one.
The load experienced by a motor will vary depending on how the hand tool is used. For example, if a hand tool is used for cutting, a load condition exists when the hand tool is cutting, and a no-load condition exists when the hand tool is not cutting.
Hand tools may generate a rotational motion or a linear motion. Both of these motions originate from the motor. In the case of rotational motion, this motion may be the direct result of the turning of the motor shaft. In the case of linear motion, frequently a cam is used to translate rotational motion of the motor shaft to linear motion. This cam can cause a varying torque load on the motor shaft, even with a constant linear output load.
Every electrical motor may be characterized as having a winding resistance and a back electromotive force (BEMF). The most common motor drive approach uses a constant voltage applied to the motor. If the load on the hand tool, and hence the motor, increases significantly, the current through and voltage across the winding resistance becomes greater. If the load becomes high enough, the required current may cause the winding voltage drop to be greater than the drive voltage, and the motor stalls. For sub-stall level loads, the motor can slow down significantly.
A motor running at a rotational speed produces a BEMF proportional to that speed. The current drawn by a motor is proportional to the output torque. The work done by the motor is equal to the output shaft rotational rate in radians per second, multiplied by the output torque in newton-meters. The current drawn by the motor is equal to the voltage across the winding resistance divided by the resistance: EQU I.sub.motor =(v.sub.supply -EMF).backslash.R.sub.winding
As more load is placed on the motor shaft, the motor draws more current. This means the voltage drop across the winding increases which in turn means that if the supply voltage is held constant, the BEMF must decrease and the motor must slow down.
As the motor slows down when it encounters a load, there is a variation in rotational speed. This variation in rotational speed is associated with a rotational acceleration. The rotational acceleration manifests itself as a vibration in the hand tool. Hand tool vibration is undesirable because it makes handling the hand tool more difficult and reduces the operator's ability to control the hand tool in a reliable fashion. Thus, in the surgical instrumentation field, it is highly desirable to reduce or eliminate vibration due to rotational acceleration.
For example, an electrical vitrectomy handpiece has a cam with a ball bearing that fits into a sinusoidal groove. When the ball bearing is in a flat part of the groove at either extreme, the load on the motor is low and the motor draws little current. When the ball bearing is at the steep part of the groove, the load is much greater and the motor may draw several hundred milliamps. As the motor rotor has significant rotational inertia, a rotational vibration is induced as it speeds up and slows down. This vibration is felt and seen by the operator/ophthalmic surgeon. If the site of the operation is near delicate tissue such as the retina of the eye, the vibration may be unacceptable to the doctor using the handpiece.
Accordingly, there is a need in the art to be able to operate a motor at a selected speed that is independent of the load experienced by the motor.
Motor speed is always proportional to the BEMF. As the load increases, the BEMF decreases, the current demanded goes up with the voltage and resistance remaining constant. Similarly, as the load decreases, the BEMF increases and the current demanded decreases with the voltage and resistance remaining constant. Because speed of the motor is always proportional to BEMF, if a way were devised to increase or decrease the drive voltage as the current demand changes to compensate for the drop across the winding resistance, the BEMF would remain constant, resulting in a constant speed. A constant speed would eliminate rotational acceleration in a handpiece and hence would eliminate vibration in the handpiece.